High-precision fluid delivery systems have become very important in many industrial applications, for example in the semiconductor industry for wafer and chip fabrication. Such fluid delivery systems typically include components such as mass flow controllers (MFCs) and mass flow verifiers (MFVs) to regulate or monitor fluid flow.
The fabrication of a single semiconductor device can require the careful synchronization and precisely measured delivery of as many as a dozen or more gases to a processing tool usually including a process chamber. Various recipes are used in the fabrication process, and many discrete processing steps where a semiconductor device is cleaned, polished, oxidized, masked, etched, doped, metalized, etc., for example, may be required. The steps used, their particular sequence, and the materials involved, all contribute to the making of a semiconductor device.
Wafer fabrication facilities are commonly organized to include areas in which gas manufacturing processes, such as chemical vapor deposition, plasma deposition, plasma etching, and sputtering, are carried out. The processing tools, be they chemical vapor deposition reactors, vacuum sputtering machines, plasma etchers or plasma enhanced chemical vapor deposition or other types of systems, machines or apparatus, are supplied with various process gases. The process gases are supplied to the tools in precisely metered quantities.
In a typical wafer fabrication facility the gases are stored in tanks, which are connected via piping or conduit to a gas box. Such gas boxes can be used to deliver precisely metered quantities of pure inert or reactant gases from the tanks of the fabrication facility to a processing tool. The gas box, or gas metering system, typically includes a plurality of gas paths having gas units. Such units typically include gas sticks which in turn can include one or more components, such as valves, pressure regulators, pressure transducers, mass flow controllers (MFCs) and mass flow meters (MFMs), as well as other units, such as mass flow verifiers (MFVs).
Prior art mass flow verifiers (MFVs) have been used to provide in situ verification of mass flow controller performance for fluid delivery systems and/or related semiconductor process tools. FIG. 1 depicts an example of one such prior art mass flow verifier (MFV) 100 as used to verify flow from a device under test (DUT), such as a mass flow controller MFC 104. MFV 100 can include a vessel or chamber 102 having a predetermined volume, an upstream or first valve 108 controlling flow between a gas manifold (not shown) and the chamber 102, a downstream or second valve 110 controlling flow from the chamber 102 to an outlet, e.g., a vacuum pump, a pressure sensor 112, typically a capacitive manometer, configured to sense the pressure within chamber 102, and a temperature sensor 114.
As shown in FIG. 1, a typical MFV 110 can include a controller 120 that receives the output signals of the pressure sensor 112 and temperature sensor 114 and controls the operation of the upstream valve 108 and the downstream valve 110.
With continued reference to FIG. 1, in operation, controller 120 is generally programmed so that during operation the controller 120 first opens the upstream and downstream valves 108 and 110 so that flow occurs through the upstream valve 108, into the vessel 102 and out the downstream valve 110. The controller 120 is further programmed so that after an initialization period sufficient to allow the flow to stabilize, the downstream valve 110 is closed to stop flow from the chamber 102. As the chamber 102 is filled with fluid from the MFC 104, the controller 120 receives measurement signals of vessel pressure from manometer 112, receives measurements of time from its clock, and determines a rate of change in vessel pressure due to the gas flow. The controller 120 then determines from these measurements the actual flow rate provided by the MFC 104 so that the accuracy of the MFC can be determined.
After the flow measurement is made, typically the upstream valve 108 is then closed and the downstream valve 110 is opened to purge the vessel 102, e.g., by way of connection to a vacuum pump (not shown). Thus, by utilizing sample values of pressure measurements, the controller 120 can calculate the gas flow rate from the measured change in pressure over time (ΔP/Δt) in the known volume of the vessel 102. An example of the operation is graphically shown in FIG. 2 represented by the mathematical models expressed in accordance with EQ. (1) described below.
FIG. 2 is a graph depicting the pressure (P) vs. time (t) relationship 200 of pressure within a typical manometer utilizing the rate-of-rise (“ROR”) measurement technique. For typical mass flow verification, a controller, e.g., controller 120 of FIG. 1, or other device/component having similar computational functionality, utilizes the rate-of-rise method of flow verification generally in accordance with the following equation:
                              Q          i                =                                                            k                0                            ⁢                              T                stp                            ⁢                              V                c                                                                    P                stp                            ⁢              T                                ⁢                      (                                          Δ                ⁢                                                                  ⁢                P                                            Δ                ⁢                                                                  ⁢                t                                      )                                              EQ        .                                  ⁢                  (          1          )                    
wherein, Qi is the average gas flow into the mass flow verifier during the period of Δt, k0 a conversion constant (=6×107 standard cubic centimeters per minute or sccm), Pstp the standard pressure (=1.01325×105 Pa), Tstp the standard temperature (273.15° K), Vc the measurement chamber volume, P the measured chamber gas pressure, and T the measured gas temperature.
While prior art mass flow verifiers (MFVs), such as shown and described for FIG. 1, may prove useful for their intended purposes, increasingly there has proven to be a need for mass flow verifiers that can operate with high accuracy at low volumetric flow rates used in high-precision fluid delivery systems. Prior art mass flow verifiers such as in FIG. 1 have proven to be unable to meet certain accuracy specifications at low volumetric flow rates, e.g., within 0.5% of an indicated flow reading error, at low flow rates at or below about 10 standard cubic centimeters per minute (sccm).
A need has arisen for verifying flow rates over larger flow ranges, e.g., 1 sccm to 10,000 sccm, at relatively low inlet pressures to the MFV, e.g., pressures approximately equal to or less than 75 Torr. Further, a single volume with multiple pressure sensors can not cover a wide flow range such as 1 sccm to 10,000 sccm due to the fact that the flow noise is amplified by the chamber volume.
What is desirable, therefore, are systems, methods, and apparatus that address the limitations noted by providing mass flow verifiers that can operate with high accuracy over a wide flow range at low volumetric flow rates.